1. Field of the Invention
This invention relates to a hypodermic injector, and in particular to a compact, needleless hypodermic injector.
The needleless injection of medicines into the body is decidedly preferable to the old fashioned needle and syringe. A needleless jet injector makes a substantially smaller hole than a needle, and consequently needleless injection is less painful than injection with a needle. Moreover, for people requiring frequent injections, e.g. diabetics, needleless injection, which results in less damage to the skin and tissue, is definitely preferable. This is an important consideration, since the areas on the body which can be used for injection are limited. However, needleless jet injectors have met with somewhat limited success.
2. Discussion of the Prior Art
Needleless jet injectors of generally the same type disclosed and claimed herein are the subject of many patents, including, for example Canadian Pat. No. 569,887, issued to R. P. Scherer on Feb. 3, 1959, and U.S. Pat. Nos. 2,928,390 issued to A. Venditty et al on Mar. 15, 1960; 3,202,151, issued to A. W. Kath on Aug. 24, 1965; 3,292,621, issued to O. H. Banker on Dec. 20, 1966; 3,292,622, issued to O. H. Banker on Dec. 20, 1966; 3,518,990, issued to O. H. Banker on July 7, 1970; 3,561,443, issued to O. H. Banker of Feb. 9, 1971; 3,688,765, issued to J. S. Gasaway on Sept. 5, 1972; 3,815,594, issued to N. R. Doherty on June 11, 1974 and 4,059,107, issued to N. Iriguchi et al on Nov. 22, 1977. Few of the devices disclosed by these patents are available in the marketplace. Many of the devices lack simplicity, i.e. are expensive to produce or use. The gas cylinders used in some of the injectors are expensive and difficult to find, and without such cylinders the devices are useless. Moreover, the pressure in such cylinders may vary. Of even greater importance is the failure of such injection devices to achieve the pressures required for proper injection. In order to effect a needleless injection properly, a high initial pressure is required to pierce the skin. Once the skin has been pierced, the pressure should drop substantailly to an injection pressure. The injection pressure should be such that all of the medicine is injected to a predetermined depth in the tissue, i.e. into the fat beneath the skin and not into the muscle. The injection pressure is determined by the back pressure in the tissue. As the medicine is injected, the back pressure in the tissue builds up to a maximum, which is substantially less than the pressure required to pierce the skin. The injection pressure should either rise with such back pressure or be maintained at a constant level at least equal to the maximum back pressure. Otherwise, there is a very real danger that the medicine being injected will be forced out of the tissue at the end of the injection step. If the injection pressure is too high, i.e. substantially greater than the maximum back pressure, the danger exists that the medicine will be injected to the incorrect depth such as into the muscle. Injections into the muscle are more painful than subcutaneous injections. With thin people, the distance between the skin and the muscle is small. Moreover, some medicines such as insulin must be injected subcutaneously; otherwise the period of relief provided by the medicine varies. In other words, when using insulin it is obviously important to know how long the effects of injection will last. The absorption time for muscle is much shorter than the absorption time for fat. By injecting subcutaneously, i.e. into the fat, a longer absorption time is obtained. If some of the medicine enters the muscle, there can be a deletereous effect.
It is worthwhile mentioning at this point that one proposal to avoid the use of needles (or for that matter injectors of the type disclosed herein) by diabetics is an insulin pump carried by the diabetic or embedded beneath the skin. While such a proposal may have its merits, for a variety of reasons including vanity, people are often loath to carry either externally or internally any contrivance which labels them as having a disease or illness of any type.
There still exists a need for a simple, yet effective needleless jet injector. The object of the present invention is to meet such need by providing a relatively simple needleless jet injector, which is capable of producing the pressure required for proper administration of a medicine. By proper administration is meant injection of the entire dose to the desired depth, i.e. subcutaneously.